The present invention is directed to a serial printer employing a wire dot printing head and, in particular, to a serial printer having a wire dot printing head where a ribbon mask is attached to and held between the end of the printing head and a platen in the printer to separate a printing paper from the printer ink ribbon.
In conventional serial printers using wire dot printing heads, a ribbon mask made of a thin material is set up in the gap defined between the end of the printing head and the platen, hereinafter referred to as "platen gap", in order to separate the printing paper from the printer ink ribbon. Such a structure serves to prevent problems caused by contact between the printing paper and the ink ribbon. However, the platen gap must be determined considering the performance of the printing head such as printing speed, duplicating capacity, durability, noise and the like so as to obtain high printing quality. Such conditions permit the platen gap to be generally between 0.4 and 0.5 mm. Such a gap is so small that precision is required in assembling and adjustment.
In order to gain such an extremely small width for the platen gap, it is usual to utilize a method wherein the platen gap is adjusted with a thickness gauge interposed between the end of the printing head and the platen in assembling. On the other hand, the set position of the ribbon mask in such a small platen gap affects the feed of the printing paper and the transportation of the ink ribbon past the printing head. For example, when the gap between the end of the printing head and the ribbon mask is smaller than the thickness of the ink ribbon, problems will be created such as unsmooth transportation of the ink ribbon or breakage thereof. Unless the gap between the ribbon mask and the platen is at least as large as the thickness of the printing paper, other problems will be created such as failure or difficulty in inserting the printing paper and unsmooth feed of the printing paper.
Accordingly, precision is required in adjusting the position of the ribbon mask in addition to adjusting the platen gap. However, in conventional serial printers, the ribbon mask is secured to the printer carriage or the printer head so that the following disadvantages are produced. First, a ribbon mask placed in the platen gap prevents the precision necessary in adjusting the platen gap, as a result of which the printing head will not accomplish its required performance. Second, ribbon masks may be fixed in several positions according to the assembler's skill, which presents the aforedescribed problems affecting printing paper feed and transportation of the ink ribbon past the printing head. Third, when a printing head or ribbon mask needs to be replaced, the aforementioned adjustments cannot be made by general users who are not skilled in adjusting the various parts and gaps. Accordingly, a printer employing a wire dot printing head which avoids the aforedescribed problems, is desired.